Successful use of lasers in surgery and material processing requires precise knowledge of focus position, focused spot size, and the peak power density of the beam. In addition, the beam characterization technique shall be quick, not require thorough alignment, and shall be simple to be operated by a non-technical personal. It also has to be low cost to be incorporated into commercial laser systems.
Present-day devices for determination of focus position, focused spot size, divergence, and quality of laser beams use knife-edges, blades and slits for scanning across the beam. The transmission of the beam past the knife-edge (or any other aperture) is then monitored for characterization of the power density distribution across the beam. Several patents, such as U.S. Pat. No. 5,100,231 to Sasnett et al, U.S. Pat. No. 4,160,598 to Firester et al, describing such approaches are incorporated by references. The disadvantages of these techniques that are using knife edges and other scanning apertures is in their low accuracy and spatial resolution. They do not allow measuring waist radius of laser beams of less than 5 microns. The apertures do not withstand high power density that is achieved in the focus of laser beams used for surgery or material processing. The process of finding the focus position is highly time consuming since it requires multiple measurements in different cross-sections of the beam in the focal region. CCD can not be used for those purposes either due to their low resolution, hypersensitivity to laser radiation and low damage threshold.
One approach of overcoming the problems outlined above relies on registering the changes in the spatial profile of the beam propagated through a transparent nonlinear optical material. Such an apparatus for power density measurement of electromagnetic radiation is suggested in U.S. Pat. No. 5,621,525 to Tabirian et al., which is incorporated by reference. The technique suggested in U.S. Pat. No. 5,621,525 to Tabirian et al. provides data only about the power density of radiation.
Thus, present devices for measuring focus position, focused spot size, divergence, and quality of laser beams are not satisfactory: they have low resolution and low accuracy due to the underlying principles of measurements; measurements are time consuming; the devices are large, heavy, and expensive which do not allow their incorporation into commercial medical or industrial laser systems.
The first objective of the present invention is to provide a technique for high-resolution and fast measurement of the focus position of focused laser beams.
The second objective of this invention is to provide means for measuring of waist radius of tightly focused laser beams.
The third objective of this invention is to provide means for fast and high-resolution measurement of divergence of laser beams.
The fourth objective of this invention is to provide simple and compact means for measurement of the quality of laser beams.
The fifth objective of this invention is to provide means for direct and high accuracy measurement of the power density of laser beams.
The sixth objective of this invention is to provide means for simultaneous high-resolution and high accuracy measurement of focus position, waist radius, divergence, beam quality, and power density of laser beams.
The invention includes focusing a laser beam, scanning a thin film of a nonlinear optical material in the focal region of the beam, and registering the changes in the intensity of the radiation at the output of the nonlinear optical material with the aid of a detector. The specific character of variation of the signal of the photodetector in time allows determination of the focus position, waist radius, and divergence of the beam with the known speed of scan. Further processing of the photodetector signal allows determination of the power density of the beam with known nonlinear optical properties of the material. Inputting the beam radius on the focusing lens allows also determination of the beam quality parameter.